In recent years, owners of oils and gas wells have learned that it is more economical to maximize the production of hydrocarbons from proven reserves than to drill new wells looking for previously undiscovered reservoirs. Specifically, it has been found in many wells that less than 50% of the hydrocarbons contained in existing proven oil or gas reservoirs are actually brought to the earth's surface.
To learn more about the underground reservoirs of oil and/or gas, for the purpose of obtaining greater oil and/or gas production, it is necessary to place sophisticated instrumentation or gauges into previously drilled or newly drilled wells. Such sophisticated instrumentation includes, but is not limited to, geophones or geometers, fiber optics, video cameras, sniffers, pressure sensors, liquid level sensors, thermometers and radiation measuring equipment.
To be able to make use of the information provided by the sophisticated instrumentation or gauges within a well, there is a need to connect the instrumentation or gauges to a recording apparatus at the top of the well.
In still other wells, electrically operated equipment such as valves or small motors are placed deep within the well bore. Reliable operation of such equipment requires communication with control apparatus at the top of the well.
Because most proven wells include a production tubing string within the casing lining the well, the communication cable connecting the sophisticated instrumentation, gauges, or electrically operated equipment to the recording or control apparatus at the top of the well is required to be positioned in the annulus between the production tubing string and the casing or between the casing and the earthen wall of the drilled hole. To assure the integrity of the communication cable it is necessary that the communication cable connecting the down-hole instrumentation or gauges, or electrically operated equipment to the recording or control apparatus at the top of the well be protected from damage by impact forces. In addition, it is necessary that the communication cable be available in long lengths as the worldwide average well depth is about 6,000 feet. Further, 25% of the oil or gas wells worldwide have a depth which is greater than 12,000 feet, and 15% of the world's oil and gas wells have a depth which is greater than 20,000 feet.
To satisfy the need for long lengths of flexible, crush resistant communication cable for connecting sophisticated instrumentation for sensing physical parameters within a well to recording equipment on the earth's surface, owners of wells have looked to various types of protected communication cables.
One type of protected communication cable considered for downhole use was the armored electrical cable used in both commercial and residential construction. This cable is often referred to as BX or Greenfield cable. The origination of BX cable can be traced back to the early U.S. patents to Greenfield--U.S. Pat. Nos. 630,502; 809,561 and 838,179. Most BX cable includes interlocking layers of sheet metal strips wrapped around electrical wires. While inexpensive and readily available, BX cable does not provide the crush resistance needed in oil wells or in hostile environments.
Another type of protected communication cable suitable for downhole use is a product referred to as "tube wire. " Tube wire is often used to convey readings from a pressure sensor at the bottom of a well to the earth's surface. The process steps illustrated in FIG. 1 are used to make tube wire. Specifically, the ends 502 of a metal jacket or sheath 500 are butt welded 504 together in the same way paper is wrapped around tobacco in a cigarette. The welded jacket is then swaged around the inner wire 506. Because of its construction, tube wire is rigid and not easily passed over small diameter pulleys or around small diameter guide blocks. In addition, frequent bending of tube wire cold works the metal jacket 500 thus increasing the rigidity of the product. Users of tube wire often experience significant problems in connecting sections of tube wire together and repairing broken sections of tube wire. Because tube wires often operate in high pressure environments, pockets of high pressure may become trapped between the inner wire 506 and the outer sheath 500 which causes the outer sheath 500 to expand to where the butt weld 504 will fail. Even though the metal tube 500 is swaged over the inner wire 506, the inner wire 506 can move longitudinally with respect to the outer sheath 500. This longitudinal movement of the inner wire 506 has actually caused the inner wire 506 to disconnect from pressure gauges at the bottom of the well. Finally, tube wire requires complex payout equipment to enable its utilization at an oil or gas well. Shown in FIG. 1A is an alternate embodiment 510 of prior art tube wire. In embodiment 510 the insulation 508 around the wire 506 has three ribs 509 which create three spaces 511 when the outer jacket 500 is swaged around the wire 506. These three spaces may be filled with epoxy or other similar material to prevent the entrapment of pressurized gas.
Yet another type of communication cable often found in oil fields is referred to as double-wound tension bearing cable. Such cables are described in U.S. Pat. Nos. 4,028,660; 4,077,022; 4,440,974; and 5,150,443. While double wound tension bearing cable is satisfactory for limited applications in shallow wells, it has been found that double wound, tension bearing cables are too inflexible and too expensive to be used in deep wells to assure communication with down-hole instrumentation gauges, or electrically operated equipment. In addition, double wound, tension bearing cable has little crush resistance and also exhibits a tendency to unwind when used in deep wells because the great weight of long lengths of hanging cable causes the outer covering to unwrap.
While cables of all shapes and sizes have been made for many years, none of the available cables met the needs of being crush resistant, manufacturable in exceedingly long lengths, able to hold an inner core of wires in position, continuously flexible, reusable, not requiring complex payout equipment to enable use, and inexpensive.
Accordingly, the inventors herein have looked to cable designs not typically used in oil wells for communication with sophisticated instrumentation or gauges. Such cables are often referred to as Bowden cables and were originally described in U.S. Pat. No. 609,570. Bowden cables are typically relatively small cables that are commonly used for push-pull force applications which require a central control wire within a coiled compression-bearing wire encasement to enable the remote application of either a push force or a pull force.
Generally, a Bowden cable is produced by spirally winding a wire at the desired lay angle (more than 45 degrees) about a central wire. Once the Bowden cable is formed, it is typically cut off in short lengths. The length of a single continuous Bowden cable that can be produced is limited by manufacturing constraints. Further, the Bowden cable must be produced in a straight condition and not coiled on a take-up reel because a take-up coiling reel would have to be rotated about a transverse axis while coiling the Bowden cable to avoid twisting the cable. If the Bowden cable were to provide the desired solution for a down-hole communication cable, the need remains to make a Bowden cable larger, crush resistant, manufacturable in exceedingly long lengths, able to firmly hold an inner core in position, able to be repeatedly flexed, reusable, easily dispensable without the use of complex payout equipment, and finally--inexpensive.